Diesel Electric Submarines Diesel generator machinery can cause a great deal of noise. Diesel electrics are only quiet when operating in electric mode.
In fact, diesel boats must raise a snorkel to have intake fresh air for combustion when operating the diesel recharging the batteries and is then visible to the world.
u212a todaro class submarine pietro venuti U212A Todaro class submarine Pietro Venuti You cannot run silently with the diesels running and charging the electrics, you can only run silently in the electric-only mode, submerged. And because that’s off batteries, your submerged endurance is limited.
Diesel electric submarines only use diesel mode when travelling on surface, or snorkeling. However, there is the Stirling engine submarine, that can run its two propulsion systems totally submerged.